There is known an assembled-type camshaft, which is rotatable in ganged relation to the crankshaft of a gasoline engine or the like, for controlling the timing to open and close the intake and exhaust valves of the gasoline engine.
The assembled-type camshaft has a shaft in the form of a metal pipe fixedly mounted in a shaft insertion hole defined in a cam piece, by means of press-fitting or the like.
One conventional method for manufacturing a camshaft of the type described above is disclosed in Japanese Laid-Open Patent Publication No. 8-90139. According to the disclosed method, a round rod of elongated steel, having a volume that corresponds to a cam to be produced, is heated to a temperature in a range from 1250 to 1280° C., and then subjected to a hot forging process by striking.
Japanese Laid-Open Patent Publication No. 2001-355709 and Japanese Laid-Open Patent Publication No. 2002-147572 disclose the technical concept of employing fine blanking to reduce a die-induced deformation, for achieving both desired forming accuracy and reducing manufacturing costs.
According to the hot forging process disclosed in Japanese Laid-Open Patent Publication No. 8-90139, an oxide layer is formed on the surface of the forged product, and further, burr wrinkles and inclusions are produced on the surface of the forged product by striking. Therefore, the forged product needs to be machined for finishing.
According to the fine blanking process disclosed in Japanese Laid-Open Patent Publication No. 2001-355709 and Japanese Laid-Open Patent Publication No. 2002-147572, since fine blanking is essentially a shearing process, it must be performed in a manner that minimizes ruptured surfaces or die-induced deformations. If a ruptured surface or die-induced deformation is formed on the surface of the produced cam, then surface pressure on the cam tends to be locally increased when the cam surface presses a valve.
The assembled-type camshaft includes a cam and a shaft, which are integrally combined with each other. If the cam were to slip against the shaft, then the crankshaft that operates the camshaft and the valve that is operated by the camshaft are brought out of synchronism with each other. In view of such a problem, Japanese Laid-Open Patent Publication No. 7-293666 proposes a technique of providing a rotation prevention mechanism in a fitting hole of the cam.
Japanese Laid-Open Patent Publication No. 11-107712 discloses a technique for fitting a cam over a hollow shaft while enlarging the shaft by plastic deformation to fix the cam in place.
The techniques disclosed in Japanese Laid-Open Patent Publication No. 7-293666 and Japanese Laid-Open Patent Publication No. 11-107712 both require a machining process for providing the rotation prevention mechanism. In addition, the rotation prevention mechanism makes the shaft and the cam complex in shape.
Japanese Laid-Open Patent Publication No. 52-50963 discloses a technique for forming a camshaft at a low cost by means of cold forging.
The technique disclosed in Japanese Laid-Open Patent Publication No. 52-50963 concerns the production of a camshaft for a brake drum, wherein the camshaft is combined with a cam in the form of a flat plate. The disclosed technique, however, is not applicable to the production of an engine camshaft, because a different type of cam is used to open and close the valves in an engine.
If the shaft and cam are separately produced and then assembled into a camshaft, then the shaft and cam need to be machined to form a rotation prevention mechanism, in order to prevent the cam from rotating relatively to the camshaft. Therefore, an additional machining process for forming such a rotation prevention mechanism is required. In addition, because the rotation prevention mechanism is incorporated therein, the shaft and the cam tend to be complex in shape.